Line of Sight Bias in Dark Matter Inferences from Galaxy Cluster Mergers

TL;DR

This paper reveals that observational biases in gravitational lensing measurements of galaxy cluster mergers lead to an overestimation of dark matter scattering cross sections by approximately 25%, due to the triaxial shape and orientation of halos.

Contribution

It identifies and quantifies a line of sight bias in dark matter cross section constraints from galaxy cluster mergers using simulations and mock observations.

Findings

Observed column density is lower than actual along the merger axis.

Weak lensing reduces the bias to about 25%.

Existing constraints are biased high by roughly 25%.

Abstract

In collisions of galaxy clusters, the lack of displacement between dark matter and galaxies suggests that the dark matter scattering depth is small. This yields an upper limit on the dark matter cross section if the dark matter column density is known. We investigate a bias in such constraints: the measured column density (along the line of sight, using gravitational lensing) is lower than that experienced by a dark matter particle, as follows. Dark matter halos are triaxial and generally collide along their major axes, yielding a high scattering column density -- but the merger is obvious only to observers whose line of sight is nearly perpendicular to that axis, yielding a low observed column density. We trace lines of sight through merging halos from the BigMDPL n-body simulation, both with and without mock observational effects. We find that a hypothetical skewer through the center of the halo along the merger axis (more precisely, along the current separation vector of the two halos) has nearly twice the column density of a typical line of sight. With weak lensing measurements, which involve some spatial averaging, this ratio is reduced to 1.25, suggesting that existing constraints on the scattering cross section are biased high by about 25%.